The invention relates to a display tube comprising a display screen, an electron gun arranged in a neck portion opposite said screen, a deflection unit having a deflection coil support comprising a field deflection coil system at its outer side and a line deflection coil system at its inner side, and an auxiliary deflection coil system comprising at least two auxiliary deflection coils arranged around the neck portion of the display tube.
The line deflection coil system is used to deflect at the line-frequency, the electron beams generated in the display tube into a first (horizontal) direction; the field deflection coil system is used to deflect, at the field-frequency, the electron beams into a second direction, transverse to the first direction.
A system of auxiliary deflection coils may be used for several reasons.
To improve the picture quality it is known, for example to subject the electron beams to scan velocity modulation (s.v.m.) during deflection by means of an s.v.m auxiliary deflection coil system. In this modulation method, the velocity during deflection of the electron beams in the horizontal direction is influenced by means of the differentiated video signal so that luminance transitions on the display screen will be sharper. It is also known to influence the deflection of the electron beams in a vertical direction in such a way that the flickering phenomenon occurring when displaying inserted symbols (such as teletext) is suppressed. Auxiliary deflection coil systems are also used in the field of convergence correction.
In a known deflection unit the two self-supporting saddle coils of an s.v.m. auxiliary deflection coil system are slid from the wide end into the cylindrical part of a deflection coil support before the line deflection coil system is arranged on its inner surface. After the deflection unit has been arranged on a display tube, the auxiliary deflection coil system is thus present between the gun-sided end of the line deflection coil system and the location where the electron beams are generated. This construction is based on the idea that the auxiliary deflection coils should be located as close as possible to the electron beams, i.e. in connection with the sensitivity they should abut the glass of the neck of the display tube.
However, practice proves that certain display tubes comprising such a system of auxiliary deflection coils for scan velocity modulation do not come up to expectations. The invention is based on the recognition that in display tubes comprising deflection units having relatively long (line) deflection coils a position of the auxiliary deflection coil systems as described above (i.e. too close to the electron gun) is not optimal. However, a position more to the front would mean that in the known construction the svm coil system must either be shortened which is not advantageous from an energetic point of view, or must be arranged completely or partly within the line deflection coils so that unwanted electromagnetic coupling giving rise to "ringing" may occur in some cases.